1. Field of the Invention
The invention relates to an ink droplet ejecting method and apparatus of an ink jet type.
2. Description of Related Art
According to a known ink jet printer of an ink jet type, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material. When the ink flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle. However, when the ink flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of printing head, multiple ink chambers are formed by partition walls of a piezoelectric ceramic material. An ink supply device such as ink cartridges, are connected to one end of each of the multiple ink chambers. The opposite end of each of the ink chambers is provided with an ink ejecting nozzle (hereinafter referred to simply as "nozzles"). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character or a figure.
An example of this type of an ink jet printer is a drop-on-demand type ink jet printer that ejects ink droplets, which is popular because of a high ejection efficiency and a low running cost. An example of a drop-on-demand type ink jet printer is a shear mode type that uses a piezoelectric material, which is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051.
As shown in FIGS. 12(a) and 12(b), this type of ink droplet ejecting apparatus 600 includes a bottom wall 601, a top wall 602 and shear mode actuator walls 603 located therebetween. The actuator walls 603 each include a lower wall 607 bonded to the bottom wall 601 and polarized in the direction of arrow 611, and an upper wall 605 formed of a piezoelectric material, t h e upper wall 605 being bonded to the top wall 602 and polarized in the direction of arrow 609. Adjacent actuator walls 603, as a pair, define an ink chamber 613 therebetween. The actuator walls 603 that are adjacent the ink chamber, in a pair, define a space 615 which is narrower than the ink chamber 613.
A nozzle plate 617 having nozzles 616 is fixed to one end of each of the ink chambers 613, while the opposite end of each of the ink chambers is connected to an ink supply source (not shown). Electrodes 619 and 621 are respectively formed on both side faces of each actuator wall 603, as metallized layers. More specifically, electrode 619 is formed on the actuator wall 603 on the side of the ink chamber 613, while electrode 621 is formed on the actuator wall 603 on the side of the space 615. The surface of electrode 619 is covered with an insulating layer 630 for insulation from ink. Electrode 621, which faces the space 615 is connected to a ground 623, and electrode 619, which is provided in each ink chamber 613, is connected to a controller 625, which provides an actuator drive signal to the electrode.
The controller 625 applies a voltage to the electrode 619 in each ink chamber, whereby the associated actuator walls 603 deform, by virtue of the piezoelectric material, in directions to increase the volume of the ink chamber 613. For example, as shown in FIG. 13, when voltage E(V) is applied to an electrode 619c in an ink chamber 613c, electric fields are generated in the directions of arrows 631 and 632 respectively in actuator walls 603e and 603f, so that the actuator walls 603e and 603f deform in directions to increase the volume of the ink chamber 613c. At this time, the internal pressure of the ink chamber 613c, including a nozzle 618c and the vicinity thereof, decreases. The applied state of the voltage E(V) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber 613. During this period, ink is supplied from the ink supply source.
Similarly, where voltage is applied to electrodes 619a, 619b and 619d in respective ink chambers 613a, 613b and 613d, electric fields are generated in respective actuator walls 603a, 603b, 603c, 603d and 603g. Each of the ink chambers 613a, 613b and 613d include corresponding nozzles 618a, 618b and 618d.
The one-way propagation time T is a time required for the pressure wave in the ink chamber 613 to propagate longitudinally through the same chamber. Given that the length of the ink chamber 613 is L and the velocity of sound in the ink present in the ink chamber 613 is a, the time T is determined to be T=L/a.
According to the theory of pressure wave propagation, upon lapse of time T, or an odd-multiple time thereof, after the above application of voltage, the internal pressure of the ink chamber 613 reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode in the ink chamber 613c is returned to 0(V). As a result, the actuator walls 603e and 603f revert to their original state (FIGS. 12(a) and 12(b)) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure, and the pressure developed by the reverting of the actuator walls 603e and 603f to their original state before the deformation, are added together to provide a relatively high pressure in the vicinity of the nozzle 618c in the ink chamber 613c, whereby an ink droplet is ejected from the nozzle 618c. An ink supply passage 626, shown in FIG. 12(b), that communicates with each of the ink chambers 613, is formed by members 627 and 628.
In this type of ink droplet ejecting apparatus 600, it is necessary to eject a small ink droplet in order to attain high print resolution. However, in printing a solid pattern by continuous dot ejection, a drop-out in white may occur, or the print density may become low, because the ink droplet is small. In the case where all of the dots formed during printing are large, the initial writing portion of a figure and fine patterns, are not attractive, or fine lines may become thick to a greater extent than necessary, thus giving rise to the problem that the print quality is deteriorated.
Japanese Published Unexamined Patent Application No. Hei 2-2008 discloses an ink droplet ejecting apparatus wherein, when a printing-free period has been detected, the electric power of a jet pulse for subsequent printing is controlled, to solve the problem of the printed image density being lowered at the initial stage of printing. However, even if such a control is made, the occurrence of a drop-out in white as noted above still remains unsolved when a solid pattern is printed using small ink droplets for effecting high resolution printing.